gro: fix truesize underestimation

skb_gro_receive() doesnt update truesize properly when adding one skb to
frag_list.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -2906,7 +2906,7 @@ int skb_gro_receive(struct sk_buff **head, struct sk_buff *skb)
 	nskb->prev = p;
 
 	nskb->data_len += p->len;
-	nskb->truesize += p->len;
+	nskb->truesize += p->truesize;
 	nskb->len += p->len;
 
 	*head = nskb;
@@ -2916,6 +2916,7 @@ int skb_gro_receive(struct sk_buff **head, struct sk_buff *skb)
 	p = nskb;
 
 merge:
+	p->truesize += skb->truesize - len;
 	if (offset > headlen) {
 		unsigned int eat = offset - headlen;